This invention relates to systems and methods for scanning a focused beam relative to a flat field, and more particularly to systems and methods for providing precise focusing of a scanning beam over a large flat field format along one axis.
Light beam scanning systems are now in wide use for various forms of printing, recording or reproduction of data or graphical information, displays and for readout of data and graphical information. Because of the capability of modern data processing systems and data reproduction systems for handling large amounts of data at high speeds, raster scan rates of the order of hundreds of lines per second are generally employed. A thin light beam source, most often a laser in modern systems, is repeatedly deflected in one direction by successive faces of a rotating polygon as the object surface is moved in the orthogonal direction by a carriage or drum. Oscillating scanning devices are also employed in such systems, although the beam scan may be used only during alternate half cycles or beam scans may alternate in direction. The scanning mechanism diverts the beam into an optical imaging system which is configured to maintain a selected degree of focus on the object surface throughout what may be a considerable scan angle and distance.
While many such light beam based writing and reading systems are in use, the scanning imaging systems heretofore employed do not fully meet the needs of the modern state of the art. Some systems are based upon the use of a collimated beam and a relatively expensive f.theta. lens system, or another complicated lens geometry. Such systems are not readily feasible or economic where it is desired to focus a beam when the spot size is very small and field very large. A number of somewhat conflicting requirements must be met if precise focusing over a wide flat field format is to be accomplished with a scanning imaging system of reasonable cost. In using a reflective scanner such as a rotating polygon scanning a noncollimated beam, a variable but predictable error is introduced during each scan because the reflective face of the polygon is spaced apart from the axis of rotation. Corrective optical systems have been devised for compensating for this variation, as well as the different path lengths that exist at different scan angles relative to a flat field. Such compensating optical systems, however, require a number of elements, and thus become very costly, particularly where large field formats (e.g. .about.17" or .intg.450 mm) are to be employed. Also, modern applications increasingly demand higher resolution, such as 2000 pixels per inc,, and the higher order of precision demands substantially more expensive optics than even prior art systems have had to use. Furthermore, refractive optics introduce problems with chromatic aberration, off-axis aberrations and light attenuation that must also be confronted and the solutions to which invariably add cost and complexity. Increased costs become disproportionate as resolution is increased, because optical and chromatic aberrations impose substantially greater penalties. A new system is needed in order to provide the needed close focusing, preferably diffraction limited, over a wide flat field without involving penalties in size, cost, or other aspects of performance.